When we have our heating turned on the boiler is noisy and you can hear the water being pumped through the radiators (annoying 'wooshing sound'). We've already bled the radiators but there is not any air in the system. We have a service contract with British Gas but this problem occurred before we started the contract. They sent someone round to quote to fix it. He suggested replacing the anti gravity valve which would cost about £200 (boiler is on floor and next to wall so would require about 3 hours work) but could not guarantee this would fix it. The noise does not occur when the hot water is on only. It is a Baxi boiler with separate hot water tank (not combi) and 6 radiators. Does anyone have any suggestions? Would flushing the system help or reducing the pump speed?